1. Field of the Invention
The present invention relates to a flyback transformer having a focus pack.
2. Description of the Prior Art
In a television receiver using a cathode-ray tube, a high voltage is required. For this purpose, a flyback transformer which generates a high voltage by utilizing a flyback pulse is used.
A flyback transformer generates and applies a high voltage (e.g., 25 kV) to the anode of a cathode-ray tube. This high voltage is also appropriately decreased across a resistor to obtain a focus voltage (e.g., 10 kV) to be applied to an electron beam focusing electrode arranged in an electron gun, and a screen voltage (e.g., 1 kV) to be applied to a screen electrode for color adjustment.
Conventionally, in order to obtain a focus voltage and a screen voltage from a high voltage generated by a flyback transformer, a unit called a focus pack is used.
Recently, Japanese Patent Laid-Open No. 2-28905 proposes a flyback transformer having a focus pack. In this apparatus, as shown in FIG. 1, an inner case 3 is fitted inside an outer case 2 to constitute a focus pack 1 as a double case. A circuit board 4, on which a resistor circuit (not shown) including a variable resistor is formed, is housed in the inner case 3. A rotational operation shaft 5 of the variable resistor extends from a shaft retaining cylinder 6 of the inner case 3 to the outside to be freely rotatable. The outer case 2 of this focus pack 1 is fitted on a transformer case 7. In this state, the transformer case 7 is filled with an insulating casting resin 8 to insulate the circuit board 4 by the inner case 3, the insulating casting resin 8, and the outer case 2. As a result, electrical discharge from a high-voltage portion of the circuit board 4 to the outside is prevented. When the rotational operation shaft 5 is rotated, the resistance of the variable resistor is changed, and the level of the focus adjusting voltage output from the circuit board 4 is variably adjusted. Reference numeral 11 denotes a low-voltage coil housed in the transformer case 7; 13, a high-voltage coil; and 15, a core.
Japanese Utility Model Laid-Open No. 2-65304 proposes a focus pack having a double case structure. In this apparatus, as shown in FIG. 2, a focus pack 1 is constituted by outer and inner cases 2 and 3, and shaft retaining cylinders 6 on the inner case 3 side, in which rotational operation shafts 5 for focus and screen adjusting variable resistors extend, are hermetically fitted in the holes in the outer case 2, thereby integrating the cases 2 and 3. In this focus pack 1, the high-voltage portion of a circuit board 4 is insulated by the double case, in the same manner as described above, thereby preventing electrical discharge from the high-voltage portion to the outside.
However, in either focus pack 1 of the double case structure proposed above, the outer and inner cases 2 and 3 are separately formed, and are integrated during assembly by fitting the shaft retaining cylinder 6 of the inner case 3 side into the hole in the outer case 2. Therefore, the outer and inner cases 2 and 3 must be formed in separate steps by using separate molds, and the cases 2 and 3 formed in the separate steps must be integrated by fitting, resulting in cumbersome procedures. As a result, the work efficiency in the manufacture of the apparatus is low, resulting in an increase in cost of the apparatus.